The clinical use of heavy particles in radiotherapy has made rapid progress during the past three years. Therapeutic trials of fast neutrons are in progress at five installations in the United States. Therapeutic applications of protons at Harvard, helium and heavy ions at Berkeley, and negative pions at Los Alamos are in progress. The project proposed here is intended to provide much needed comparative physical and radiobiological data to evaluate the complementary therapeutic potential for fast neutrons, protons, helium and heavy ions, and negative pions. The proposed radiobiology studies are of RBE and OER determinations using cultured cells (V79), cell-survival measurements with depth of penetration (T-1 cells), recovery from sublethal and potentially lethal damage in cultured cells, cell-cycle response studies in Chinese hamster cells (line CHO), and early and late reactions and residual injury studies in the mouse foot. Dosimetry and microdosimetry relevant to the proposed radiobiology studies will be performed. Results will be reported in a format useful to radiotherapists interested in the relative effectiveness of heavy particles under comparable therapeutically relevant physical conditions, in particular with similar widths for the Bragg peak.